The Dumont Dunes near Death Valley are a popular playground for off-road vehicle fans, who come here -- sometimes in large groups -- to race their machines up and down the sandy slopes. Early this morning, an enthusiastic group gathered in the southwest section of the dunes watching an off-road vehicle do just that. But there was something unusual about this group, and about the vehicle they'd brought with them. It wasn't a run-of-the-mill ORV they piloted through the Dumont Dunes. This vehicle, or its twin, anyway, is about to go as far off-road as a vehicle can get. The group was from Pasadena's Jet Propulsion Laboratory, and their ride was dubbed "Scarecrow," a sibling of the Mars Rover Curiosity set to land on Mars in late summer.

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In early August, as part of the Mars Science Laboratory mission launched last November, an orbiter will drop Curiosity inside Gale Crater on the Martian equator. Mission planners are aiming for a landing site on an oddly colored patch of soil that scientists believe may be an ancient dry lake, near Mount Sharp, which seems to possess rock layers similar to those laid down in bodies of water on earth. Areologists hope to find signs of microbial life, learn more about the long evolution of the Martian landscape and atmosphere, and gather other data that may help eventual human missions to the planet.

Mars Science Laboratory is part of NASA's Mars Exploration Program, managed by JPL. Mission staff at JPL expect Curiosity to encounter terrain very similar to terrestrial dunes in Gale Crater, so they brought Scarecrow to the Dumont Dunes to give rover operators a little more data to aid in negotiating the sand traps of the Red Planet.

Curiosity (and Scarecrow, for that matter) is about ten feet long: twice the length of the largest previous Mars rover. The Mars-bound version has a mast-mounted system with a two-megapixel still- and video camera, as well as what JPL is calling the "Chem-Cam," an innocuous name for a laser that vaporizes rock samples from up to 30 feet away, then analyzes the vapor and dust to determine its chemical makeup. It also sports a forward robotic arm, with tools for handling and manipulating samples of Martian rock and soil, and then analyzing them for chemical makeup and signs of life. All of this is supported in insectoid fashion on a chassis with six long articulated legs, with wheels at the ends about a foot in diameter. Curiosity's operators designed the wheel treads to include a repeating pattern, so that scientists examining images from Curiosity can gauge distances by counting the treadmarks across the surface. That repeating pattern is a bit of old-style school spirit: "JPL" in Morse Code.

There are a few major differences between Curiosity and Scarecrow. With a mass of around 900 kilograms, Curiosity weighs in, on Earth, at just over 1,900 pounds. Mars gravity is a bit more than one third Earth's, so Scarecrow's handlers pared its weight down to around 750 pounds to better simulate interaction between the rover and the sandy terrain expected for Curiosity. That means no camera equipment, no robot arm, and no ChemCam to vaporize suspicious rock samples at the Dumont Dunes.

That's okay: JPL staff have enough work to do just seeing how Scarecrow moves. As the rover turns painstakingly slowly on a stretch of dune with a mild slope, JPL roboticist Jaret Matthews watches data accumulate on a laptop screen. "We don't have GPS on Mars, so we won't be able to pinpoint Curiosity's location as precisely as we could here on Earth," says Matthews. "The rover knows how many degrees it's turned and how much its wheels have rotated, but that doesn't necessarily translate to how much the rover will actually have moved, especially on sandy surfaces."

To prepare for this, the JPL crew has surrounded Scarecrow with half a dozen motion capture cameras, essentially the same technology used to convey lifelike motion in animated films. Odd grey globular protrusions the size of softballs stick up from Scarecrow's frame, giving the cameras something to resolve.

"We're taking Scarecrow to different sections of the dunes with differing slopes, and collecting information on how it actually moves in all those different conditions," Matthews says. "That will allow us to anticipate how Curiosity will move in similar conditions on Mars."

However Curiosity moves, it will do so slowly. Its designers have given it -- and its earthbound twin -- a top speed of about 0.02 miles per hour. To put that into perspective, the distance an average person could walk in a leisurely minute would take Curiosity or Scarecrow more than an hour and a half to cover. It's hard to imagine just how slow that really is until you see it for yourself:

It's not the the kind of speed one generally thinks of in the context of space exploration, but given the distance between Mars and Earth, and the consequent radio communications lag between Earth and Curiosity's onboard systems, it makes sense. Were a Martian to jaywalk in front of the rover JPL wouldn't see it for anywhere from four to 20 minutes, depending on our planets relative positions, and it would take just as long for Curiosity to know JPL had hit the brakes in response.

So Curiosity will move slowly, and engineers have given the rover a bit of navigational autonomy to make up for the delay introduced by the speed of light. "We're not going to be joysticking this one," laughs Matthews. "It'll be more like us telling Curiosity 'We want you to be over there in 24 hours,' and Curiosity will figure out the best way to get there in the interim. And if it encounters an unanticipated navigational hazard, it's programmed to take the most prudent action in the circumstances, including just stopping and waiting for new instructions."

Curiosity's landing August 6 will kick off an on-ground mission at least one Martian year long. (That's 687 of your Earth days.) In that time mission staff expect Curiosity will cover up to a dozen miles of ground -- very, very slowly. Right now, Curiosity and the Mars Science Laboratory are approaching Mars at about 13,000 miles per hour. After a trip at speeds like that, a slow saunter around the surface of Mars will likely be a nice break.

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